Because of an increasing number of reports describing Paramyxovirus-like structures in MS brains, attempts have been made to immunolabel, under EM, SSPE viral antigen in active plaque of 3 MS cases. Results have been negative so far although specific label was obtained in animal brains using similar techniques. After formaldehyde fixation, an intracellular localization of the immunoglobulin IgG has been repeatedly observed in the active MS plaques. Besides the plasma cells, reactive astrocytes and their gliofibrilles also react with antihuman IgG. The meaning of this observation has to be further explored by immunological methods.